Single Semiconductor Quantum Dots Research Articles

Steady state optical gain in small semiconductor quantum dots

The optical gain phenomena in a single semiconductor quantum dot as well as in an inhomogeneous distribution of quantum dots have been studied. The analytical results obtained using the density matrix technique and numerical analysis for CdS quantum dots show an enhancement of gain in the presence of biexcitons. The numerical results show larger gain in bigger quantum dots. A redshift of gain maxima occurs when the distribution width is large. The realization of high gain quantum dot lasers and photodetectors appears to be a distinct possibility.

Exciton complexes in individual quantum dots as a single-photon source

We investigate the photon emission statistics of recombination processes of different exciton complexes confined in a single photo-excited semiconductor quantum dot. The complexes are identified by the photoluminescence spectrum of the dot, which shows a rich line structure due to the various combinations of electrons and holes that can be confined in the dot. We demonstrate single-photon emission from each of these exciton complexes, revealing the biexciton state to produce single photons with significantly less jitter in their emission time than the single exciton.

Local Strain Effects in Near-Field Spectra of Single Semiconductor Quantum Dots.

ABSTRACTExperimental and theoretical investigations of high-energy shifts of single InAs, InGaAs, InAlAs and InP quantum dot (QD) emission lines induced by contact pressure exerted by a near-field optical fiber tip are reported. “Pressure” coefficients of 0.65–3.5 meV/nm have been measured for ground state emission lines in agreement with numerical calculations. We show that the observed increase of the tip-induced energy shift with increasing aperture diameter is caused by a decrease of the uniaxial strain component. We also report the effect of emission instability of single QD emission intensity under tip-induced pressure.

Mechanical Interaction in Near-Field Spectroscopy of single Semiconductor Quantum Dots.

AbstractWe have studied high-energy shifts of single quantum dot (QD) emission lines induced by contact pressure exerted by a near-field optical fiber tip. “Pressure” coefficients of 0.65-3 meV/nm have been measured for self-organized InAs/GaAs, InAs/AlAs and InP/GaInP QDs in agreement with numerical calculations of the local strain field. We found an increase of the tipinduced pressure with increasing aperture diameter from 50-300 nm. A correlation between the shift rate and QD stiffness was obtained. We also observed an order of magnitude increase of single QD emission intensity with increased pressure.

Semiconductor quantum dot: a quantum light source of multicolor photons with tunable statistics.

We investigate the intensity correlation properties of single photons emitted from an optically excited single semiconductor quantum dot. The second order temporal coherence function of the photons emitted at various wavelengths is measured as a function of the excitation power. We show experimentally and theoretically that a quantum dot is not only a source of nonclassically correlated monochromatic photons but is also a source of multicolor photons with tunable correlation properties. We found that the emitted photon statistics can be varied by the excitation rate from a sub-Poissonian one, where the photons are temporally antibunched, to super-Poissonian, where they are temporally bunched.

Optical absorption in semiconductor quantum dots: Nonlocal effects

The optical absorption of a single spherical semiconductor quantum dot in an electrical field is studied taking into account the nonlocal coupling between the field of the light and the polarizability of the semiconductor. These nonlocal effects lead to a small size anf field dependent shift and broadening of the excitonic resonance which may be of interest in future high precision experiments.

Single-photon emission from exciton complexes in individual quantum dots

Applications in optical quantum information technology require a new type of light source able to emit exactly one photon periodically with minimal timing jitter. We investigate the photon emission statistics of different electron-hole recombination processes in a single photo-excited semiconductor quantum dot. We demonstrate single photon emission from each of these exciton complexes, revealing the biexciton state to produce single photons with significantly less jitter in their emission time than the single exciton state.

Nonlinear absorption in semiconductor quantum dots

The density matrix approach has been employed to understand the mechanism of nonlinear absorption / gain in a single semiconductor quantum dot as well as in an inhomogeneous distribution of quantum dots. The analytical results show an enhancement of excitonic gain in the presence of biexcitons. The same has been confirmed by the numerical analysis carried out for a realistic system of semiconductor quantum dots of CdS embedded in a glass matrix. The numerical study exhibits a red shift of maximum gain at large distribution widths.

Optical spectroscopy of single quantum dots at tunable positive, neutral, and negative charge states

We report on the observation of photoluminescence from positive, neutral and negative charge states of single semiconductor quantum dots. For this purpose we designed a structure enabling optical injection of a controlled unequal number of negative electrons and positive holes into an isolated InGaAs quantum dot embedded in a GaAs matrix. Thereby, we optically produced the charge states -3, -2, -1, 0, +1 and +2. The injected carriers form confined collective 'artificial atoms and molecules' states in the quantum dot. We resolve spectrally and temporally the photoluminescence from an optically excited quantum dot and use it to identify collective states, which contain charge of one type, coupled to few charges of the other type. These states can be viewed as the artificial analog of charged atoms such as H$^{-}$, H$^{-2}$, H$^{-3}$, and charged molecules such as H$_{2}^{+}$ and H$_{3}^{+2}$. Unlike higher dimensionality systems, where negative or positive charging always results in reduction of the emission energy due to electron-hole pair recombination, in our dots, negative charging reduces the emission energy, relative to the charge-neutral case, while positive charging increases it. Pseudopotential model calculations reveal that the enhanced spatial localization of the hole-wavefunction, relative to that of the electron in these dots, is the reason for this effect.

“On”/“off” fluorescence intermittency of single semiconductor quantum dots

Single molecule confocal microscopy is used to investigate the detailed kinetics of fluorescence intermittency in colloidal II–VI (CdSe) semiconductor quantum dots. Two distinct modes of behavior are observed corresponding to (i) sustained “on” episodes (τon) of rapid laser absorption/fluorescence cycling, followed by (ii) sustained “off” episodes (τoff) where essentially no light is emitted despite continuous laser excitation. Both on-time and off-time probability densities follow an inverse power law, P(τon/off)∝1/τon/offm, over more than seven decades in probability density and five decades in time. Such inverse power law behavior is an unambiguous signature of highly distributed kinetics with rates varying over 105-fold, in contrast with models for switching between “on” and “off” configurations of the system via single rate constant processes. The unprecedented dynamic range of the current data permits several kinetic models of fluorescence intermittency to be evaluated at the single molecule level and indicate the importance of fluctuations in the quantum dot environment.

Linewidth Broadening of Quantum Dot Emission Caused by Temperature Fluctuations

The temperature dependence of the homogeneous linewidth of optical transitions in a single semiconductor quantum dot (QD) due to temperature fluctuations is calculated. Since the temperature fluctuations are inversely proportional to the considered volume and the QD volume is very small, the temperature of a QD and therefore its energy states are not well defined. Therefore, the emission and absorption spectra are homogeneously broadened. The broadening is calculated for simplified cuboid-like electron and hole wavefunctions assuming spatially constant material parameters. The linewidth is strongly dependent on temperature and volume of the QD, whereas the shape of the wavefunction is less important. For cubic In 0.5 Ga 0.5 As QDs with volume (5 nm) 3 , the broadening is ∼10 μeV below 100 K and reaches ∼1.4 meV at room temperature.

Monitoring single scattering events in single quantum dots

The quantum-jump approach is used for a theoretical description of resonance luminescence from a single semiconductor quantum dot in contact with its solid-state environment. For continuous excitation of the single-exciton groundstate the luminescence exhibits bright periods, where photons are spontaneously emitted from the exciton decay, which are interrupted by dark periods when one electron or hole suffers a spin flip. It is shown that continuously monitored resonance luminescence provides a very sensitive measure of such rare single scattering events in quantum dots.

Observation of Charged Few-Particle States in the Optical Spectra of Single Semiconductor Quantum Dots

A detailed discussion of the optical properties of single n-type modulation-doped semiconductor quantum dots is presented. We use the photo-depletion/back-hopping mechanism to optically control the number of surplus electrons in the dot. Comparison of the experimental data with calculated luminescence spectra, obtained within a direct-diagonalization approach for calculating electron–hole states, allows the identification of luminescence from the decay of up to fivefold charged excitons. Finally, we study the influence of fluctuating local electric fields, produced by ionized impurities in the surrounding of the dots, on the optical selection rules and on the linewidth of the emission peaks.

Observation of Charged Few-Particle States in the Optical Spectra of Single Semiconductor Quantum Dots

A detailed discussion of the optical properties of single n-type modulation-doped semiconductor quantum dots is presented. We use the photo-depletion/back-hopping mechanism to optically control the number of surplus electrons in the dot. Comparison of the experimental data with calculated luminescence spectra, obtained within a direct-diagonalization approach for calculating electron-hole states, allows the identification of luminescence from the decay of up to fivefold charged excitons. Finally, we study the influence of fluctuating local electric fields, produced by ionized impurities in the surrounding of the dots, on the optical selection rules and on the linewidth of the emission peaks.

Local absorption spectra of single and coupled semiconductor quantum dots

We study theoretically the local absorption spectra of single and double semiconductor quantum dots (QDs), in the linear regime. The three-dimensional confinement leads to an enhancement of the Coulomb correlations, while the spectra depend crucially on the size of the ‘local’ probe. We show that because of such Coulomb correlations the intensity of certain optical peaks as a function of the resolution can exhibit an unexpected non-monotonic behavior for spatial resolutions comparable with the excitonic Bohr radius. We finally discuss the optical near-field properties of coupled QDs for different coupling strengths.

Dynamics of Excitons in Single Semiconductor Quantum Dots Probed by Time-Resolved Optical Spectroscopy

We resolve spatially, spectroscopically and temporally the photoluminescence emission from single self-assembled In(Ga)As/GaAs quantum dots, The temporal evolution of the emission spectrum after pulsed excitation is measured for various excitation intensities at various ambient temperatures. The evolution of the spectrum with the increase in both steady state and pulse excitation intensities is measured as well. A multi-exciton model is used for calculating the temporal and excitation intensity dependence of the measured spectra. The quantitative agreement between the measured and calculated spectra provides an unambiguous determination of the radiative lifetime of a single quantum dot exciton. This lifetime is 4-6 ns long and is temperature independent. The reduced spatial coherence between the confined exciton and the radiation electromagnetic field quantitatively explains this long radiative time.

Correlation between fluorescence intermittency and spectral diffusion in single semiconductor quantum dots.

We find a correlation between the dynamics of fluorescence intermittency and spectral diffusion in the spectroscopy of single CdSe nanocrystal quantum dots (QD). A statistical analysis of the data suggests two populations of blinking events: blinking followed by large spectral diffusion shifts and blinking with small or no spectral shifts. Although unexpected from earlier studies, the correlation between blinking and spectral shifting is consistent with a model of QD ionization as the mechanism for the blinking event, followed by a redistribution of local electric fields that results in spectral shifting.

Carrier-Carrier Correlations and Their Effect on Optically Excited Single Semiconductor Quantum Dots

We resolve spatially, spectroscopically and temporally the photoluminescence emission from single self-assembled In(Ga)As/GaAs quantum dots. The rich photoluminescence spectrum and its evolution with time after pulse excitation and with the density of excitation is experimentally measured and analyzed using a theoretical multiexciton model. From the quantitative agreement between the measured and calculated spectra, the radiative lifetime of a single quantum dot exciton is unambiguously determined.

A close look on single quantum dots

Quantum dots, often referred to as artificial atoms, open the field of quantum resolved spectroscopy to semiconductor physics. The current article is designed to review the field of interband optical spectroscopy on single semiconductor quantum dots.

Carrier-carrier correlations in an optically excited single semiconductor quantum dot

We applied low-temperature diffraction-limited confocal optical microscopy to spatially resolve and spectroscopically study photoluminescence from single self-assembled semiconductor quantum dots. Using selective wavelength imaging we unambiguously demonstrated that a single photoexcited quantum dot emits light in a few very narrow spectral lines. The measured spectrum and its dependence on the power of either cw or pulsed excitation are explained by taking carrier correlations into account. We solve numerically a many-body Hamiltonian for a model quantum dot, and we show that the multiline emission spectrum is due to optical transitions between confined exciton multiplexes. We furthermore show that the electron-electron and hole-hole exchange interaction is responsible for the typical appearance of pairs in the photoluminescence spectra and for the appearance of redshifted new lines as the excitation power increases. The fact that only a few spectral lines appear in the emission spectrum strongly indicates fast thermalization. This means that a multiexciton relaxes to its ground state much faster than its radiative lifetime.